In general, a table look-up method is well known as a method of automatically setting a beam alignment of an electron optical system (referred to as EOS, hereinafter) of an scanning electron microscope. In this method, optimum parameter values of the EOS are previously registered for each constant acceleration voltage, and the EOS parameters corresponding to an acceleration voltage to be set are calculated and interpolated on the basis of the registered data. In addition, an alignment method using a Faraday cup is adopted as an automatic control method. In this method, the irradiation current is read by use of a Faraday cup, and an exciting current for controlling an alignment coil is determined under closed loop control so that the irradiation current value can be maximized. In this case, an initial excitation current passed through the alignment coil is determined by interpolating the values obtained in accordance with a table look-up method.
In the above-mentioned table look-up method, however, a data file of optimum parameter data to be previously registered must be updated according to the EOS conditions. In more detail, whenever an electron gun or a movable aperture of the electron optical system is exchanged, the data file must be updated. Further, it is also necessary to update the data file according to the lapse of time. In addition, when the beam alignment is effected according to any given acceleration voltage, interpolation calculations are required, with the result that the number of optimum parameter data increases with increasing range in which the acceleration voltage is to be set. For instance, when a voltage range between 0.5 and 2.0 kV is interpolated at 100 V intervals, 16 optimum parameter data files of 100 V steps must be prepared in total. Otherwise, there exists a problem in that the alignment discrepancy from manual operation increases.
In the case of the gun alignment method of using a Faraday cup, the tilt correction is effected by obtaining the maximum value of the beam current. However, the shift correction must be effected by determining the quality of the obtained secondary electron image (for instance, by observing the change in contrast on the image), in addition to the beam current. In the case of the lens alignment method of using the Faraday cup, on the other hand, it is required to find the maximum beam current conditions by setting the Faraday cup at the sample position. In this case, however, since the sensitivity of change in the beam current is low relative to the change in excitation of a lens alignment coil, although it is possible to obtain a maximum beam current on the basis of the determined alignment coil excitation value, there exists a case where the obtained excitation value will not match the optimum alignment condition.
In summary, in the conventional electron beam alignment methods, it has been so far difficult to reduce the alignment error automatically.